Static discharge is a naturally occurring electrical phenomenon. Triboelectric charge, often referred to as static electricity, can build up in common materials to some degree, and is eventually discharged as the charge traverses a path toward an electrical ground. Static electricity buildup is usually strongest in insulative materials because they discharge so slowly.
Circuit boards contain microcircuitry which can be easily damaged by electrostatic discharges of relatively small magnitude. Magnitudes of human body potential as small as 50 volts can permanently damage these devices. For comparative purposes, to illustrate the extreme sensitivity of microcircuits to this phenomenon, a visible charge from a human hand to a door knob in winter will often exceed 10,000 volts. Thus, extreme caution must be taken in protecting such microcircuit components from electrostatic charge.
To provide protection from electrostatic discharge for packaged articles, a principle of physics referred to as the Faraday cage effect is often employed. Electricity does not penetrate a conductive enclosure. The static electric charge will go around the enclosed space, distributing itself to produce an equipotential surface with zero electrical field inside. By surrounding a static sensitive article with a conductive enclosure, the article is shielded from electrostatic fields and discharges originating outside of the enclosure.
In my co-pending U.S. patent application Ser. No. 07/336,733, I have disclosed a multiple-ply anti-static paperboard product for use in Faraday cage-type packaging of electrostatically sensitive articles, such as semi-conductor components and electronic circuit boards. The multiple-ply paperboard product comprises a layer of high-carbon content paperboard sandwiched between two layers of static-dissipative material. Such multiple-ply paperboard is particularly useful in the manufacture of dividers, pads and walls of containers for packaging articles which are required to be shielded from exposure to static discharge.
The packaging material described in the above-identified application has been found to be very beneficial in the protection of articles against damage by static electricity, but in many instances, that material affords more protection against certain types of static electricity and less protection against other types of static electricity than is required for specific applications. Specifically, and as one example, many applications require only protection against static electricity generated internally of the package either by an electrical component or article rubbing against a portion of the package or by one portion of the package rubbing against another portion of the package. In applications of this type, there is no need for the electrical conductivity of the center ply of this multiple-ply anti-static material.
It has therefore been an objective to provide a multiple-ply package material which may be utilized in the manufacture of a container or portion of a container and which provides effective protection against damage to products or articles packaged in the container resulting from static electricity to which the package may be subjected.
One attempt at protecting static electricity sensitive articles against static electricity is disclosed in U.S. Pat. No. 4,658,985, to McNulty. This patent discloses a bag having two plies of anti-static (polyethylene) material and an electrically conductive fabric or mat embedded therebetween to provide a shield from electrostatic discharge for a bagged article. Because the bag is lined with an anti-static material, it affords protection against electrostatic charge generated internally of the package, but the bag does not, by itself, provide adequate rigidity for physical protection of a microcircuit component. In order to provide physical protection, the bagged article must be placed within another cell in a rigid container. This results in additional material and material handling costs requiring a bag enclosure, additional labor costs associated with bagging the component or article, and additional shipping costs due to reduced packaging density. Reduced packaging density is due to the fact that each cell of a container which must hold an article within a bag, rather than just the article itself, occupies more space than the article alone. Thus, for a container having a given volume, use of bag packaging necessitates larger size cells, resulting in the packaging of fewer articles per container.
Another technique for protecting static electricity sensitive articles against static electricity is disclosed in U.S. Pat. No. 4,623,594 to Keough. This patent discloses that a mixture of prepolymer and anti-static agent may be applied to a substrate, such as polypropylene fiber or paper or glass, and then cured to the anti-static agent by contacting the mixture with electron beam radiation. Because the mixture is cured in situ, after application to the substrate, this technique is very limited in its application and, to date, has only been applied commercially to bag materials and vacuum formable sheets of plastic. While this treatment does provide some protection against generation of static electricity, it is very expensive and is not particularly suitable for protective materials which are either all dissipative or all conductive.
Additionally, for physical protection, articles protected against static electricity by material treated in accordance with the disclosure of this patent must be enclosed within a bag, and that bag, if it is to be physically protected, must be packaged in another rigid container. This again results in additional material and material handling costs, as well as reduced packaging density.
Heretofore, anti-static materials have also been applied to a cardboard substrate, but that cardboard substrate-applied anti-static material, usually low-density polyethylene, has been rendered anti-static by being doped with a chemical anti-static additive, generally some form of amine. Unfortunately, amines have several undesirable characteristics when used to impart anti-static properties to packaging materials. First of all, amines do not render the material to which they are added permanently anti-static. Rather, that anti-static coating of material loses its anti-static property over a period of time. Otherwise expressed, that amine-doped material has a relatively short shelf life because the amines, trapped within the plastic, evaporate or outgas with time from the plastic or other material within which the amine is trapped such that the plastic loses its anti-static property. Secondly, the amine may be very corrosive to some metals, including the metals from which many microcircuits are manufactured. Consequently, the amine, outgassing from the anti-static plastic, can, and often does, corrode and impair the surface electrical characteristics of the component which the anti-static material is intended to protect. Additionally, the amines, and more particularly the chemicals which result from amine breakdown, will often outgas from the anti-static plastic and chemically attack the polycarbonate upon which many microcircuits are applied. In the course of attacking the polycarbonate, the chemicals produced by amine breakdown often cause stress cracks, and ultimately failure. Additionally, the electrical properties of the amines contained in the anti-static plastic are humidity dependent, meaning that the amine-doped plastic intended to afford protection to articles contained in a package of the amine-containing anti-static material is not effective in an atmosphere having a certain minimum humidity level. In some atmospheres, such as those which are very dry, as in dry areas of the United States, amine-containing anti-static plastic materials may not be suitably anti-static. While charge production may be somewhat reduced in these drier areas, it is not reduced enough to substantially eliminate the risk of damage.
Another anti-static coating may be in the form of an acrylic or other polymeric spray. However, such sprays are not permanently anti-static/static dissipative, and usually those sprays are also hygroscopic. That is, they may combine with water vapor and cause undesired water deposition on the surface of the board.
Another concern relates to the environmental and economical impact of packaging materials, which are often discarded after a finite number of uses. It would be environmentally and economically advantageous to manufacture packaging material from components that are more durable and thereby can be reused more often, made with recycled materials, recyclable themselves, can be safely incinerated or are naturally degradable. This helps the environment and eliminates the ever-increasing costs of special handling and/or waste disposal associated with non-recyclable material.
For packaging materials such as paperboard, the word "recyclable" generally means that the components are repulpable for subsequent use in other paper or paperboard articles. The phrase "naturally degradable" generally means that the material breaks down into nonhazardous, constitute compounds or elements within a reasonable time under normal environmental conditions. For instance, photodegradable materials are broken down by the effects of ultraviolet rays from sunlight. Some materials may oxidize and wear away after prolonged exposure to the atmosphere. For the purpose of this application, materials that are biodegradable, ultraviolet light degradable, or oxidizable are considered naturally degradable.
In other packaging applications, biodegradable materials have been used in loose form within packages, in lieu of styrofoam. Biodegradable materials in pliable form, such as bags, have also been used.
However, prior to the invention described in this application, applicant was unaware of any other successful attempt to provide a relatively rigid packaging material developed to specifically address both the concerns of protecting packaged articles from static electricity and minimizing adverse impact on the environment.